make sensorfw plugin handle the sensord restarting.

 This way, clients that are connected will continue to receive updates
 after the sensord has restarted, without the need to restart the
 sensor.

Task-number: QTBUG-34050
Change-Id: I4a8fbca189764cf23a1453feae5c1fd9ed6911e7
Reviewed-by: Thomas McGuire <thomas.mcguire@kdab.com>